Echo is a common problem in telecommunications systems, and can be very annoying to users of telecommunications systems having relatively long transmission delays, as for example satellite telecommunications systems and cellular radio telecommunications systems. Consequently, several methods for suppressing echo have been developed.
In many radio telecommunications systems, available radio spectrum is limited. In such systems, low bit rate encoding of speech signals is used to reduce the bandwidth required for transmission of each speech signal and to increase the number of speech signals that can be transmitted over the available radio spectrum. LPC coding of speech signals is used in many such systems.
Known methods for suppression of echoes in signals encoded using LPC-based compression are generally performed at the encoder or decoder where the original speech signal or the synthesized speech signal is available for processing. This location for echo suppression functionality may not be convenient in some applications, particularly where LPC codecs in the network are bypassed (e.g. in mobile-to-mobile connections) to provide better end-to-end voice quality (see, for example, U.S. patent application Ser. No. 08/358,949 filed Dec. 19, 1994 in the names of Hermon Pon et al and entitled System for TDMA Mobile-to-Mobile VSELP Codec Bypass).
Accordingly, there is a need for echo suppression functionality that can be applied to LPC-encoded speech signals at locations other than the codecs without excessive degradation of end-to-end speech quality.